Zulip and others can only be used within one team, we want to build a system that can be used for all conversations, hence the fallback emails sent to people not yet on plum mail.

We also are trying to choose features that encourage less frequent, more productive conversations, not be just another chat interface

Also, its trivial to have multiple teams on the same Zulip org, people are just subscribed to different sets of streams. So, say you have two teams: mobile and desktop, for example. You can have streams prefixed with that name:

#mobile-hangout #mobile-issues #mobile-support

OR

#m-hangout #m-issues #m-support

Good conventions for stream naming makes it really natural for multiple mostly isolated teams to exist on the same Zulip org, and share some streams like #announce, #general, etc that everyone is subscribed to.